Posterior and anterior tooth restoration is typically achieved by excavating decayed tooth structure and filling the resulting cavity with a paste-like filling material, which is then hardened by chemical or photochemical curing processes. Resin based dental restorative materials are becoming the material of choice by dentists and patients due to desirable esthetic properties. Tooth colored resin based composite materials are usually composed of dispersions of inorganic filler particles in a polymerizable organic resin matrix. Most commonly, especially in direct restorations, the restorative material is cured by exposure to actinic radiation.
Stress bearing restorations, such as those involving the occlusal surface of posterior teeth, require the use of mechanically strong, highly filled restorative materials to withstand the forces resulting from mastication. Such restorative materials are typically highly viscous, which makes accurate placement of the restorative difficult and highly technique sensitive. Inadvertently, the cavity may be insufficiently filled and adaptation of the restorative material to the cavity walls may be incomplete, resulting in gaps between the restoration and the tooth structure, which can lead to increased sensitivity, intrusion of fluids and bacteria, and can result in continued tooth decay and premature failure of the restoration. Less highly filled, flowable restorative materials, on the other hand, facilitate proper adaptation but lack the required strength for stress bearing restorations. Moreover, since these less highly filled materials tend to flow under their own weight, they cannot be shaped to conform to the original tooth anatomy.
When using highly filled restorative materials to restore a deep cavity, the material is typically placed incrementally in thin layers. Each incremental layer is cured individually before placing the subsequent increment to counteract both polymerization shrinkage stress and low light penetration depth and thus incomplete hardening of the restorative. Restoring a tooth using the layering technique is therefore relatively time consuming and also increases the risk of leaving voids between the layers, which could significantly weaken the restoration.
It is therefore desirable to provide a highly filled, paste-like restorative material having a high viscosity that can be lowered to a liquid-like flowable consistency when dispensed into a cavity by using an external stimulus and that the initial paste-like viscosity is restored upon removal of said external stimulus to facilitate shaping to the proper contour. Furthermore it is desirable for this material to exert low polymerization shrinkage stress to the restored tooth and to exhibit sufficient depth of cure to enable placement and adequate hardening of fewer but thicker layers of the restorative material. Ideally, the entirety of such a restorative material required to fill the whole cavity would be placed and hardened in bulk.
It is known that particulate dispersions with high solids content, of which dental restorative composites are examples, typically exhibit shear-thinning and, in some cases, thixotropic behavior and that their viscosity can be lowered through the action of vibrations, including sonic or ultrasonic vibrations. For example, U.S. Pat. No. 5,244,933 discloses dental compositions having a viscosity too high to be workable for the intended purpose, which can be rendered workable by exposure to oscillations. When used clinically for restoring a tooth defect, however, such otherwise unworkable materials require the use of special oscillating equipment throughout each manipulation step, which makes their use cumbersome and thus presents a disadvantage to the practitioner. Prior art paste-like resin based restorative materials, including universal composite materials such as Herculite® (Kerr Corp., Orange, Calif.) can be dispensed at a reduced viscosity through the use of special vibration-assisted dispensers. For example, U.S. Pat. No. 7,014,462 discloses a method and a device for introducing a dental filling material into a tooth cavity, where the device subjects the filling material to the action of vibrations as is it injected into the cavity. However, the ease and extent of viscosity reduction of a given material through the action of sonic or ultrasonic vibrations varies and is highly dependent on the material's composition. Moreover, since vibration-induced liquefaction is accompanied by heat generation due to internal friction, simply increasing the power or duration of the vibrating action to improve efficiency can lead to a substantial temperature rise, which could potentially harm the tooth. For these practical reasons, the degree of liquefaction of prior art dental restoratives is limited and, particularly for very highly filled materials, is insufficient to achieve the liquid-like behavior of a flowable restorative material.
In summary, there is a need to provide a highly filled dental restorative material that offers high strength for load bearing restorations, low polymerization shrinkage, and high depth of cure, yet readily liquefies to a flowable-like consistency through activation by vibration energy to greatly simplify clinical placement.